Effect of source compositions on adakitic features: A case study from the Buya granite, in western Kunlun, NW China

Abstract

Characterized by high Sr/Y (> 40) and La/Yb (> 20) ratios, the adakitic features are, generally, attributed to a garnet-bearing residue. However, adakitic features may be inherited from an adakitic source. In this contribution, we take the Buya granite as an example to demonstrate the effect of source on adakitic features through petrology, zircon U-Pb geochronology and Lu-Hf isotopes, whole-rock geochemistry, and Sr-Nd-Pb isotopes. The 445 Ma Buya granite contains many coeval microgranular magmatic enclaves (MMEs), and some late fine-grained granitic dikes. The host granite and the MMEs display sub-parallel to parallel trace-element patterns characterized by enrichments in LREE and LILE, and depletions in HREE and HFSE, typical of adakitc features. Importantly, the two lithologies possess comparable zircon Hf and whole rock Sr-Nd-Pb isotopic compositions that plot along a mantle array. The MMEs are interpreted as cognate fragments of early-formed crystals and the parental magma was sourced from overriding mantle wedge metasomatized by fluids derived from the subducted Proto-Tethys slab. Comparatively, the dikes, with positive εNd(t) (∼2) values and lower (87Sr/86Sr)t (∼0.705), were products of the partial melting of a plagioclase-rich crustal source. Although the host granite and the MMEs possess adakitic features, the latter have higher La/Yb ratios and relatively lower Sr/Y ratios, which are inconsistent with garnet effect because of similarly high DY/DSr (∼1900) and high DYb/DLa ratios (∼2300) for garnet/melt. Instead, this feature is attributed to the fractional crystallization of hornblende and allanite. Therefore, the host granite and the MMEs may inherit adakitic features from the source. Likewise, the adakitic features of the dikes are attributed to a plagioclase-rich source due to the enrichment in Sr element. The early fractional crystallization of amphibole and biotite in the MMEs can elevate Zr/Sm and lower Nb/Ta ratios, respectively, in the residual liquid to form evolved magma similar to adakites and TTGs. The Buya granite displays an arc affinity, demonstrating the northward subduction of the Proto-Tethys oceanic plate.